RC: Stub agent access in RC component spec

[p-datadog]

What does this PR do?

Stubs agent access in a unit test.

Motivation:
The test is failing if I have no local agent running.

Change log entry
None

Additional Notes:
This PR is part of my investigation of skipped tests on JRuby - the remaining ones deal with RC barriers.

How to test the change?

Existing CI

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 100.00%
• Overall Coverage: 95.18% (-0.00%)

View detailed report

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[pr-commenter]

Benchmarks

Benchmark execution time: 2026-01-06 21:03:13

Comparing candidate commit a17b95f in PR branch rc-component-spec-mocks with baseline commit 53b2275 in branch master.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 43 metrics, 2 unstable metrics.

scenario:profiling - gvl benchmark samples

• 🟥 throughput [-805.125op/s; -794.514op/s] or [-6.187%; -6.106%]